The recording density of magnetic recording apparatuses has been improved recently. In investigating defects or other features of a magnetic-field-producing component of such an apparatus, it is desirable to observe its surface topography with nanometer-scale accuracy and to observe near the surface the magnetic field that is produced by the component. In investigating causes of an attribute of the magnetic field, it is most desirable to observe both simultaneously.
Martin et al. proposed magnetic force microscopes (MFMs) using the principle of atomic force microscopy in the following documents:
(1) Y. Martin and H. K. Wickramasinghe, "Magnetic imaging by force microscopy with 1000 resolution," Appl. Phys. Lett., 50 (20), pp. 1455-1457, May. 18, 1987.
(2) P. C. D. Hobbs, D. W. Abraham and H. K. Wickramasinghe, Magnetic force microscopy with 25 nm resolution," Appl. Phys. Lett., 55 (22), pp. 2357-2359, Nov. 27, 1989.
These conventional MFMs vibrate a cantilever supporting a tip made of a magnetic material near its resonant frequency, and optically observe shifts in vibration amplitude which occur when the tip detects a magnetic force, by using an interferometer or similar instrument. As data related to the magnetic field, the first and second derivatives of the magnetic field can be obtained. The same apparatus can be used to observe the topography of a surface of a sample by causing the tip to sense the atomic force acting between an atom at the end of the tip and an atom on the sample surface, instead of the magnetic force. Thus the apparatuses in both (1) and (2) above allow not only the magnetic force, but also the surface topography to be observed after being translated into the form of forces acting on the tip end. However, it is difficult to observe both the magnetic force and the topography simultaneously, since the effects of the two forces must be distinguished in the observed amplitude shift. In addition, it is troublesome to set the optics for optical observation of the cantilever amplitude.
In addition to the difficulties in adjustment and operation discussed above, MFMs are not yet commercially available. It would be desirable for an apparatus to allow a magnetic field to be observed by using scanning tunneling microscopy, since the scanning tunneling microscopes are commercially available and relatively easy to adjust and operate. The following documents discuss approaches in which an STM is used to observe both the topography of a surface and the force acting on a tip of the STM simultaneously:
(3) N. A. Taubenblatt, "Lateral Forces and topography using the scanning tunneling microscope and optical sensing of the tip position," IBM Technical Disclosure Bulletin, Vol. 32, No. 3A, pp. 250-251, August 1989.
(4) U. During, J. K. Gimezewski and D. W. Pohl, "Experimental observation of forces acting during scanning tunneling microscopy," Phys. Rev. Lett., Vol. 57, No. 19, pp. 2403-2406, November 1986.
The apparatus described in (3) above shares a problem with the MFMs discussed above in that changes in amplitude of the STM tip are sensed optically. Further, the apparatus described in (3) it is not suitable for observation of a magnetic field produced by a magnetic head because it can detect only a force vibrating the tip in a direction parallel to the sample surface. In the apparatus described in (4) above, as shown schematically in FIG. 9, a thin gold film sample 100 whose topography is to be measured is placed on the end of a flexible cantilever 102. The cantilever 102 is vibrated with a small amplitude of about 0.25 .ANG. near its resonant frequency by thermal excitation, which vibration is sensed in form of a component of the frequency of a tunneling current. When a gradient of the van der Waals force is present and acts between the end of a tungsten STM tip 104 and the sample surface, the resonant frequency of the cantilever 102 varies. Data on the van der Waals force can thus be obtained by analyzing the tunneling current with a spectrum analyzer and detecting shifts in the resonant frequency. Data on the sample topography are obtained in the form of signals applied to the z-direction driving means for the tip while the distance between the tip 104 and the sample surface is kept constant, on the basis of the tunneling current from which high-frequency components are eliminated.
However, even if the tip of the apparatus of FIG. 9 is made of a magnetic material, it is difficult to observe both the surface topography and the magnetic field of a sample when the sample is a thin-film magnetic head, for the following reasons:
(I) A magnetic head is usually attached to a slider. Further, it is coupled to a wire that supplies a current to produce a magnetic field. Therefore, the weight of a magnetic-head sample is typically much greater than that of a gold thin film. It is difficult to prepare a flexible cantilever that can support such a heavy sample.
(II) For a variation in the tunneling current caused by a force to be distinguished from one caused by the topography, the vibration frequency of the sample borne on the cantilever must be high. However, it is difficult to vibrate the cantilever at a high frequency while it is carrying a heavy sample such as a magnetic head. Even if it can be vibrated at high frequency, it would be difficult to detect a shift in the resonant frequency, because the vibration amplitude will be small.
(III) Since the sample and a sample carrier, which have much greater weight and volume than the tip, vibrate, the measurement is apt to be affected by noise, and the signal-to-noise (S/N) ratio of the measurement result is consequently decreased. In this instance noise can be air vibrations such as wind, sound waves, and the like, and/or insulator vibration.
In short, it is unnatural to move a sample that has a large volume and mass when the sample is to be observed with a small STM tip. There is accordingly a need for a measuring apparatus capable of obtaining data on both the topography and the magnetic field simultaneously, without moving the sample.
It is therefore an object of the invention to provide a novel apparatus capable of observing a magnetic field near a sample surface by means of a simple manipulation in an STM mode for measuring a tunneling current while scanning a tip across the sample surface.
A further object of the invention is to provide an apparatus and a method capable of simultaneously observing both the topography of a sample, even if the sample is heavy, and a magnetic field just above the surface, in an STM mode.
Another object of the invention is to allow observation of a magnetic field with a slightly modified conventional STM.